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The difference in ventilation heterogeneity between asthmatic and healthy subjects quantified using hyperpolarized ³He MRI

¹Department of Radiology, Brigham & Women's Hospital, Boston; ²Department of Biomedical Engineering, Boston University, Boston; and ³Department of Radiology, University of Massachusetts Medical School, Worcester, Massachusetts

Submitted 23 October 2007 ; accepted in final form 17 November 2008

In this pilot study, algorithms for quantitatively evaluating the distribution and heterogeneity of human ventilation imaged with hyperpolarized (HP) ³He MRI were developed for the goal of examining structure-function relationships within the asthmatic lung. Ten asthmatic and six healthy human subjects were imaged with HP ³He MRI before bronchial challenge (pre-MCh), after bronchial challenge (post-MCh), and after a series of deep inspirations (post-DI) following challenge. The acquired images were rigidly coregistered. Local voxel fractional ventilation was computed by setting the sum of the pixel intensity within the lung region in each image to 1 liter of inhaled ³He mixture. Local ventilation heterogeneity was quantified by computing regional signal coefficient of variation. Voxel fractional ventilation histograms and overall heterogeneity scores were then calculated. Asthmatic subjects had a higher ventilation heterogeneity to begin with (P = 0.025). A methacholine challenge elevated ventilation heterogeneity for all subjects (difference: P = 0.08). After a DI postchallenge, this heterogeneity reversed substantially toward the baseline state for healthy subjects but only minimally in asthmatic subjects. This difference was significant in absolute quantity (difference: P = 0.007) as well as relative to the initial increase (difference: P = 0.03). These findings suggest that constriction heterogeneity is not a characteristic unique to asthmatic airway trees but rather a behavior intrinsic to all airway trees when provoked. Once ventilation heterogeneity is established, it is the lack of reversal following DIs that distinguishes asthmatics from non-asthmatics.

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